Methods of making dental restorations



eadsets Paw 913 3,247,293 I Patented Apr. '39, Q66

3,247,293 METHODS OF'MALKINGDENTAL RESTORATIONS Richard'L. Myerson, Newton, M3SS.,.EISSlgliQI to Myerson Tooth Corporation, Cambridge, Mass., a corporation of Massachusetts No Drawing. Filed'Dec. f4, 1962,Ser. No. 242,086 '5 Claims. (Cl.'264-1'6) The present invention relates to methodsof making artificial ceramic dental restorations such as teeth, jackets, crowns, inlays, etc. More specifically, it relates to methods of making artificial ceramic dental restorations having a small void volume therein. I

In United States Patent No. 2,724,166, there is described a method of reducing the 'void volume of ceramic teeth by applying superatmospheric pressure to the teeth during firing in a pressure furnace after the ceramic materials have fused'sufficiently toform a non-porous skin and While such materials are fiowable and maintaining such superatmospheric pressure on the teeth during cooling to solidification. The superatmospheric pressure compresses the gas trapped within the teeth When the continuous skin 'is formed. After the ceramic portions have solidified the compressed gases cannot expand when the pressure is relieved. Preferably, thecontinuous skm is formed'during a' preceding firing and cooling step. The pressure on the teeth immediately preceding and during the formation of the continuous skin may be atmospheric or sub-atmospheric but in any event is less than the superatmospheric pressure referred to above.

It has been discovered that the void volume of ceramic dental restorations can be reduced by the use of superatmospheric pressure to compress the gas bubbles without maintaining such pressure on the teeth duringcooling to thereby eliminate the nec essity of (1') keepingthe teeth in the pressure furnace during cooling and (2) cooland reheating the furnace for each batch. Consequently, the pressure furnace can be continuously maintained at a single elevated temperature, the superatmospheric pressure being released only to insert and remove the teeth from the pressure furnace. Thus, the pressure is applied, maintained and released while the tooth is at the same elevated temperature in the pressure furnace. This substantially reduces the total time of the teeth in the pressure furnace to thereby increase the capacity of the furnace. Since pressure furnaces are relatively expensive, this is an important advantage. Furthermore, the cost of operating the pressure furnace is also reduced because it is not necessary to reheat the furnace for every batch.

Elimination of the necessity of maintaining superatmospheric pressure on the teeth during cooling is based on the discovery that due to the high viscosity of the ceramic materials of the teeth at relatively low firing temperatures, e.g. 1900 F. to 2100 F., the rate of compression of the gas bubbles by the superatmospheric pressure and the rate of expansion of such bubbles when the pressure is released are relatively slow so that an interval of time is required, during which the superatrnospheric pressure is applied, to compress the gas bubbles to adequately reduce the void volume and substantially the same time interval is required at the same temperature and hence the same viscosity for the compressed bubbles to expand to their original volume after release of the superatmospheric pressure. This interval of time increases with increase in viscosity and the viscosity increases with decrease in temperature.

Accordingly, by applying superatmospheric pressure to the teeth at a temperature at which the viscosity of the ceramic material is high during a time interval sufficient to adequately compress the gases and by releasing the pressure at such temperature and subsequently cooling the teeth from such elevated temperature to a temperaturejat which the ceramic material is solidified beforethe compressed gases have had a chance to expand appreciably, the void volume of the resulting tooth is comparable to the void volume obtained when the pressure is maintained on the tooth'during the cooling step.

Thus, reduction'in void volume is achieved by controllingthe ratio of the time interval at the elevated temperature during which the superatmosphe'ric pressure is applied to the teeth and the time interval after release of the superatniosphericpressure at such temperature and during which the teeth are cooled from such temperature toa temperature below'that atvvhichl the compressed gases can expand, the latter timelintervalordinarily being substantially shorter than the former'time interval so that the ceramic material has solidified before the compressed gas has had a chance to expand appreciably. The shorter the latter time intervalas compared to'the former time interval, the less the compressed gas volume will expand and consequently the better the results. It, has been 'found that such latter time intervalis suificiently short to achieve excellent results where the teeth are removed from the hot pressure furnace and cooled in the atmosphere immediately after the pressure isreleased.

r The length of thetime interval during which the superatmospheric pressure is applied 'toadequately 'reduce the size of the bubbles will depend on the temperature, the superatmospheric pressure and the'nature of the ceramic material. The higher the temperature, the higher the pressure and the lower theviscosity of the material, the less Will be the time interval necessary. By the same token, with higher temperatures and with materials having less viscosity for any given temperature, a shorter maximum cooling time after release of pressure is required in order to prevent the compressed gases from expanding too much before the temperature of the ceramic materials is reduced to a temperature at which the ceramic material is solidified and hence the compressed gases cannot expand and further. H 7

As aforesaid, temperatures in the nature of from 1900 F. to 2100 F. are preferred. With such temperatures the teeth may be conveniently cooled to a temperature at which the compressed gasescan no longer expand without any appreciable expansion of the compressed gasesmerely by removing them ,(the teeth) from the hot pressure furnace and cooling them in the atmosphere promptly after pressure release. Although higher temperatures can be used, the viscosity of the material is reduced so much that it is difficult to prevent the compressed gases from expanding too much after the pressure is released and before the teeth can be adequately cooled. On the other hand, below 1900 F. for materials commonly used for artificial teeth relatively long time intervals under pressure are required which reduces the capacity of the pressure furnace.

Pressures as high as atmospheres or more and as low as 5 atmospheres or less may be used. If lower temperatures are used, the same compression time interval can be used by increasing the pressure and Vice versa.

The ceramic materials of the teeth are conventional, e.g. feldspar, kaolin, silica, etc., and may be the same as those described in my US. Patent No.2,724,166.

If desired, opasifying agents may be used to control translucency of the teeth. Without them, the void volume may be reduced to such an extent that the teeth may become too transparent unless opacifying agents are used.

The void volume and the pressure thereof in the finished teeth can be controlled for any given ceramic material by controlling the magnitude of the superatmospheric pressure and the elevated temperature and the ratio of the compression and cooling time intervals. Thus, with greater pressures the gas bubbles can be compressed to a .3 smaller volume and with greater elevated temperatures they, mb o pr e as allg iy lum w the same pressure in a shorter time. By increasing the cooling time interval as compared to the compression interval the gases will expand to a greater volume before the ceramic material solidifies.

ExampLe k 1 A number of raw, green ceramic teeth biscuits are formed with a ceramic enamel simulating portion and a ceramic denture simulating portion in a conventional manner by molding, baking and heating to burn out the binder (see U.S.Patent No. 2,230,164 to Myerson).

The green teeth are fired in a continuous kiln in a conventional manner at atmospheric pressure, the heating cycle comprising bringing the teeth to a temperature of 2400 F. (high gloss producing temperature) in about one hour and allowing them to cool at a similar rate. The ceramic materials forming the outside'of the biscuits fuse to form a glossy, non-porous, gas impervious skin. 1

The resulting teeth are then placed in a pressure furnace maintained at a temperature of 1950 F. The furnace is closed and the pressure in the furnace raised to 100 pounds per square inch. After approximately one and hour at 100 p.s.i. and 1950 F., the pressure is quickly released (the temperature is still 1950 F. at the time of pressure release), the door of the furnace opened and the teeth removed and cooled. The resulting teeth have an enamel with a much smaller void volume and much greater translucency than the same teeth fired only in the first kiln and fired in both the kiln and furnace but without the use of super-atmospheric pressure in the furnace. "Upon grinding the glazed surfaces'the resulting surfaces are much less pitted also. The void volume, translucency and ground surfaces compare with those of teeth made in accordance with U.S. Patent No. 2,724,166. Example 2 This example-is the same as Example 1 except that the temperature in the pressure furnace is 1900- F. and the teeth are kept in the pressure furnace at this temperature and at 120 psi. for four hours.

It is pointed out that the lower the temperature in the pressure furnace and hence the greater the viscosity of the ceramic material, the less is the expansion of the compressed gases after the pressure is releasedand the teeth are removed from the furnace and cooled'and the better are the results.

On the other hand, the lower the temperature the longer the teeth must be retained in the pressure furnace to adequately compress the, gases and the less the capacity of'the furnace.

Although with lower temperatures the time of pressure application to adequately, compressthe. gases may be reduced by increasing the pressure, the use of higher pressures requires a stronger. furnace.

The embodiment of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for producing dental restorations comprising a ceramic portion, said method comprising at least two firing andcooling steps, one of said firing steps comprising heating said ceramic portion at least until the ceramic materials thereof have fused sufficiently to form a substantiallycontinuous surface of fused material, the second of said firing and cooling steps being subsequent to said one firing and cooling step and comprising applying 'superatmospheric pressure to said ceramic portion while it is at an elevated temperature during said second firing step for a time sufl'lcient to compress the void volume within said ceramic portion to thereby reduce the size thereof, releasing said superatmospheric pressure while said ceramic portion is at said elevated temperature and subsequently cooling said ceramic-portion from said-elevated temperature to a ternperature at which said ceramic portion is solidified-before said compressed volume has had an opportunity to expand to its original volume, said elevated temperature of said second firing step beingbelow that temperature at which said com pressed 'void volume will expand to its original volume after release of saidsuperatmospheric pressure and before said solidification occurs, said superatmospheric pres sure being greater than the pressure at which said ceramic portion is maintained immediately preceding and during the formation of saidcontinuous surface. 2. A method according to claim 1, said elevated temperature being not substantiallygreater than about 3. A method according to claim 2, said elevated temperature being between about 1900 F. and 2000 F.

' 4. A method according to claim 1, said first firing step being carried out under a vacuum. I

5. A method according to claim 1, said first firing" step being carried out at atmospheric pressure.

References Cited by the Examiner ALEXANDER H. BRODMERKEL Primary Examiner.

J. R. DUNCAN, B. SNYDER, Assistant-Examiners. 

1. A METHOD FOR PRODUCING DENTAL RESTORATIONS COMPRISING A CERAMIC PORTION, SAID METHOD COMPRISING AT LEAST TWO FIRING AND COOLING STEPS, ONE OF SAID FIRING STEPS COMPRISING HEATING SAID CERAMIC PORTION AT LEAST UNTIL THE CERAMIC MATERIALS THEREOF HAVE FUSED SUFFICIENTLY TO FORM A SUBSTANTIALLY CONTINUOUS SURFACE OF FUSED MATERIAL, THE SECOND OF SAID FIRING AND COOLING STEPS BEING SUBSEQUENT TO SAID ONE FIRING AND COOLING STEP AND COMPRISING APPLYING SUPERATMOSPHERIC PRESSURE TO SAID CERAMIC PORTION WHILE IT IS AT AN ELEVATED TEMPERATURE DURING SAID SECOND FIRING STEP FOR A TIME SUFFICIENT TO COMPRESS THE VOID VOLUME WITHIN SAID CERAMIC PORTION TO THEREBY REDUCE THE SIZE THEREOF, RELEASING SAID SUPERATMOSPHERIC PRESSURE WHILE SAID CERAMIC PORTION IS AT SAID ELEVATED TEMPERATURE AND SUBSEQUENTLY COOLING SAID CERAMIC PORTION FROM SAID ELEVATED TEMPERATURE TO A TEMPERATURE AT WHICH SAID CERAMIC PORTION IS SOLIDIFIED BEFORE SAID COMPRESSED VOLUME HAS HAD AN OPPORTUNITY TO EXPAND TO ITS ORIGINAL VOLUME, SAID ELEVATED TEMPERATURE OF SAID SECOND FIRING STEP BEING BELOW THAT TEMPERATURE AT WHICH SAID COMPRESSED VOID VOLUME WILL EXPAND TO ITS ORIGINAL VOLUME AFTER RELEASE OF SAID SUPERATMOSPHERIC PRESSURE AND BEFORE SAID SOLIDIFICATION OCCURS, SAID SUPERATMOSPHERIC PRESSURE BEING GREATER THAN THE PRESSURE AT WHICH SAID CERAMIC PORTION IS MAINTAINED IMMEDIATELY PRECEDING AND DURING THE FORMATION OF SAID CONTINUOUS SURFACE. 